PROJECT SUMMARY/ABSTRACT The ventrolateral periaqueductal gray (vlPAG) plays an important role in descending pain modulation. The GABA disinhibition hypothesis proposes that tonic GABAergic neurotransmission at the level of the vlPAG serves to inhibit output excitatory projections, regulating descending analgesic mechanisms. Disinhibition of vlPAG excitatory neurons that project to the rostral ventromedial medulla (RVM) is thought to allow subsequent activation of RVM cells that will project to the dorsal horn of the spinal cord and inhibit nociceptive information processing, resulting in analgesia. Altered vlPAG neural transmission, characterized by a hypoglutamatergic and enhanced GABAergic neurotransmission, is thought to contribute to the development and maintenance of chronic pain. In an attempt to understand this circuit, pharmacology and electrical stimulation of the vlPAG have partially described its role in descending pain modulation, but due to the lack of cell-type specificity, the identity and definitive role of the neurons responsible for descending analgesia remains unclear. Techniques such as chemo- and opto-genetics, in combination with genetic mouse models, allow us to selectively manipulate vlPAG neuronal populations and finally interrogate the role of these in nociceptive processing. Preliminary data demonstrates that we can bidirectionally modulate sensory thresholds via manipulation of this circuit under nave conditions. Our findings support the hypothesis of a local tonic GABAergic control over vlPAG neurons. We hypothesize that reduction of the vlPAG GABAergic tone or stimulation of output glutamatergic neurons that project to the RVM will result in attenuation of thermal and mechanical hyperalgesia, in addition to attenuating spontaneous pain, under a persistent inflammatory pain model. Preliminary results demonstrate that chemogenetic inhibition of local GABAergic (Vgat) or optogenetic stimulation of Vglut2 RVM-projecting vlPAG neurons results in attenuation of inflammation-induced thermal and mechanical hyperalgesia. In brief, the proposed research aims to characterize the descending PAG circuitry at both an anatomical and molecular level and will identify the role of vlPAG GABA and glutamate neurotransmission and vlPAG-RVM projections in persistent inflammatory pain. A precise understanding of vlPAG-RVM circuitry, neuronal subpopulations and the mechanism by which the vlPAG can modulate persistent inflammatory pain may direct future research focused on novel targeted therapies for chronic inflammatory pain.